implement defining C variadic functions

[Vexu]

Based on #6272

Closes #515

[InKryption]

Casual bikeshed: would it make sense to prefix the builtins with "c", like @cImport and co? So it would be @cVaStart and @cVaCopy. I don't necessarily think that looks great, but I think it would make sense from a consistency pov.

[lambdadog]

Casual bikeshed: would it make sense to prefix the builtins with "c", like @cImport and co? So it would be @cVaStart and @cVaCopy. I don't necessarily think that looks great, but I think it would make sense from a consistency pov.

Definitely agree with this. It should be made very clear to the user that they're an antipattern when writing Zig and should be used only for compatibility reasons.

As a note, is it possible to limit vararg usage to callconv(.C) functions? That would, IMO, be the ideal.

[Vexu]

As a note, is it possible to limit vararg usage to callconv(.C) functions? That would, IMO, be the ideal.

It already is limited, look at the added compile error test.

[lambdadog]

It already is limited, look at the added compile error test.

Ah, my apologies. I missed that bit.